Hello all.... possible CGE adopter here


fetoma
 

Hi Group,

I'm new here doing some due diligence reading on adopting the OnStep into my CGE mount which I believe has a bad motor board. I'm not an electronics wiz by any means, but I am a machinist and a mechanic that has had many mounts taken apart and put back together for hypertuning under my belt, and have done some modifications also.

For those that have integrated the OnStep system into their CGE mounts, I think I've seen three of them, do you have any pointers or best direction to take? I saw someone mention a 1:1 pulley ratio that might be nice.

Thanks,
Frank


Khalid Baheyeldin
 

On Wed, Dec 18, 2019 at 09:24 PM, fetoma wrote:
I saw someone mention a 1:1 pulley ratio that might be nice.
That was probably me.

The rationale is as follows:

Most users had to drill the housing of the CGE in order to make room for the larger
pulley that will go on the worm shaft. The larger pulley is a must to get the needed
gear reduction.

One way to avoid this space constraint is to use a motor with a gearbox, with 10:1 or
15:1 reduction. That reduction gives more torque and finer resolution. And once you
use a gearbox the pulleys can be 1:1 which should fit without drilling the housing.

The gearbox will has some backlash, but that may not be an issue since PHD2 will
calculate and compensate for it anyway.

On the other hand, since you are an experienced machinist, you can just go with the
proven solution others have done.

One simpler way of attaching the motor is through a perforated metal strip, the
type used for furnace duct work. I can't find the link to the mount that had it.



fetoma
 

Thanks Khalid. I want to try and stay away from gears and stay with a belt. I'm guessing that they drilled the bottom of the housing to accommodate the large pulley?


Khalid Baheyeldin
 

On Wed, Dec 18, 2019 at 09:41 PM, fetoma wrote:
I want to try and stay away from gears and stay with a belt. I'm guessing that they drilled the bottom of the housing to accommodate the large pulley?
Yes.
You can see pictures and video on the Showcase page.
Search for Aisling, and follow the links there.


George Cushing
 


tnut55
 

I read an article on the Machine Design website comparing planetary vs spur gear reducers used in Pittman servomotors.  It basically stated that planetary gears were noisier, more costly but had about half the backlash of spur gears reducers. 

I took this into account when I ordered steppers for my CGE (not yet installed) from stepper online.  Mine are NEMA 14 which have about the same torque characteristics of the original Pittman motors the best I can tell and are physically similar in size.  Unfortunately, because they are NEMA 14, they are not available with the precision planetary reducers.  I settled on 14HS20-1504S-PG5 with regular planetary reducers and a stated <= 1deg backlash in the reducer.

I am surmising that the backlash is as good as the original Pittman and perhaps better.  I plan on using a 1:1 transfer from the motor/reducer to the worm just so I will not have to modify the casting.  The worm gear bracket will have to be modified in any case.  There are other reductions available but 5:1 seems more than adequate.

On Thursday, December 19, 2019, 2:06:58 PM CST, George Cushing <pinyachta@...> wrote:



Khalid Baheyeldin
 

Anecdotal info: the MPJA Vexta NEMA11 with 18:1 gear reduction has the following backlash:
RA: 117"
DE: 33"

All in all, they are 0.5 arc minute and ~ 2 arc minutes. No degrees there at all.

These are spur gears, but the motor was made in Japan, so maybe better than average?
Also, no idea why they are so different, nor which one is the typical value.

I was guiding with backlash enabled. On my list to do, if the sky ever clears, is to test
the "guide disable backlash" parameter in OnStep and see if there is any difference.


George Cushing
 

Forgot to say to ignore the claim that the gear can't be removed. Mild heat (350°C) frees it or brute force works too. Here's my gear puller. Nut is an ETX-125 Az retainer and the bolt a 1/4-20 Meade LX200 fork arm bolt. Push-pin is a broken 1/8" bit.


Khalid Baheyeldin
 

On Thu, Dec 19, 2019 at 04:33 PM, George Cushing wrote:
Forgot to say to ignore the claim that the gear can't be removed. Mild heat (350°C) frees it or brute force works too.
For the MPJA motors, Dave held the pulley that came with it in a vise, then hammered on the shaft until the pulley was out.
No ill effect observed on 4 motors.


Howard Dutton
 

On Thu, Dec 19, 2019 at 01:33 PM, George Cushing wrote:
Mild heat (350°C) frees it or brute force works too.
First set I made a puller for too and had at it, was a struggle but worked out.  Second set I carefully heated the little gear with a propane torch... it basically fell right off.

Seems like a heck of a lot of mount for these little motors though, I'd go bigger.